Detection circuits are known, which include a photodiode detector, which produces a current proportional to the intensity of incident light. The circuits further include a transimpedance amplifier circuit, which converts this current into an output voltage. For example, U.S. Pat. No. 5,030,925 shows a detection circuit of this type.
A recognised problem with detection circuits of this type is that the input light signal intensity, and hence the photodiode current, which is proportional thereto, can vary over a very wide dynamic range. For example, it may be necessary to handle input signals which vary over several orders of magnitude. Without special attention to the design values of the impedances at the amplifier input, the wide dynamic range of inputs to the amplifier would cause problems in producing output signals which are sufficiently large to be useful at low signal levels, while still producing output signals for large input signal levels which the circuit can handle without saturating or limiting.
U.S. Pat. No. 5,708,392 discloses a detection circuit with a limiting transimpedance amplifier (TIA) circuit, in which the amplifier circuit has a feedback resistor with a diode coupled across the resistor. Thus, at high signal levels, the diode conducts the input current, and limits the output voltage to a level below that which would cause saturation of the output stage of the amplifier. TIA amplifiers which use diodes across their input/output must handle the overload current in their output stage, and this requires larger transistors with more capacitance. Thus these designs are more complex and require more silicon area when fabricated in an integrated circuit.
Further, EP-A-0745868 discloses a detection circuit having a photodiode detector and a transimpedance amplifier. In order to be able to handle excessive input currents during an initial detection phase, the optodiode is connected to ground through a further diode, and is also connected to a positive voltage supply through an RC network.